Hey there,
I was reading information about this and noticed the following:
Ideally you will use Unity to develop your game. Due to its large user base it is easier for us to get support if needed and the majority of the games we have published in the past were developed using Unity. Also, it is generally easier for us to export Unity games to a variety of platforms. Importantly, Unity supports exporting to HTML5. Browser-based games are often more accessible because they can be played, for example, in public libraries and at schools. We strive to publish games that are accessible.
Does that basically mean that people will give you the source code of the project, and it’s up to you to compile it and share it to other platforms?
On that note, do you have any policy for games made with a custom engine?